Tie the beef with twine so it stays round. Rub it with salt and black pepper. Heat oil and butter in a large pan. Brown the meat on all sides very quickly. Remove it and let it cool completely.

Place the mushrooms in a food processor. Pulse them until they are chopped very finely. Cook them in the same pan with wine. Stir until all the liquid has evaporated away.

Stir the heavy cream into the mushroom mix. Let it cook for one more minute. This makes a thick mushroom paste called duxelles. Move it to a bowl to cool down.

Lightly flour your counter or a large board. Roll out the puff pastry into a rectangle. It must be large enough to wrap the beef. Make sure the pastry is still cold.

Spread the mushroom mixture over the cold pastry. Leave a small border around the very edges. If using pâté, spread it on the beef first. This adds an extra layer of rich flavor.

Remove the twine from the cooled beef tenderloin. Place the beef in the center of the pastry. Fold the sides of the dough over the meat. Seal the edges with a little cold water.

Wrap the whole Wellington tightly in plastic wrap. Put it in the fridge for 30 minutes. This helps the pastry keep its shape later. Chilling is a secret to a perfect crust.

Preheat your oven to 400 degrees. Place the roll on a baking sheet. Brush the top with the beaten egg wash. Bake for 45 minutes for a medium-rare center.